bitbake: bitbake: Unbuffer stdout for log files

bitbake: bitbake: Unbuffer stdout for log files

It is possible to lose critical log data when python exits in an
unorderly fashion via segmentation fault or certain types of crashes.
This is because the buffer characteristics are inherited from the top
level stdout, which should be set to unbuffered, for the purpose of
all the forked children.

This pushes the buffering to the OS, instead of having python managing
the buffers in its stream handler class.

This change is also to provide the ability to tail logs written from
processes in "real time" because they would be written in an orderly
fashion depending upon the OS characteristics for the file I/O.

(Bitbake rev: c6a367bc3224adafca698a4ffc5414ad83842c16)

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>